This invention relates to a pigment dispersant, particularly to a pigment dispersant including a carboxylic acid-functional first polymer and a poly(ethylene oxide/propylene oxide) block second polymer. Also the invention relates to a method for dispersing a pigment in an aqueous medium using the pigment dispersant, an aqueous pigment dispersion including the pigment dispersant, and an aqueous ink including the pigment dispersion.
Pigment is suspended in an aqueous medium along with a pigment dispersant and subjected to input mechanical energy, commonly called milling or grinding the pigment, in order to form a pigment dispersion. A pigment dispersant capable of facilitating the formation of a variety of pigments to provide stable pigment dispersions in which the pigment is provided substantially at the primary particle size of the pigment with the highest pigment loading possible at a desired viscosity is sought.
U.S. Pat. No. 6,008,270 discloses ink jet inks containing block copolymers of polyethylene oxide and polypropylene oxide; it is further disclosed that the block copolymers may be added to the mill grind.
U.S. Pat. No. 5,172,133 discloses an ink comprising a water-soluble resin and a liquid medium; the water-soluble resin contained as a dispersing agent for the pigment preferably has a weight average molecular weight from 3,000 to 30,000 and may contain copolymerized carboxylic acid monomer.
Nonetheless, improved pigment dispersants are still desired in order to provide pigment dispersions having a higher pigment loading at a given viscosity and/or exhibiting increased pigment dispersion viscosity stability, preferably exhibiting compatibility with ink letdown vehicles and formulation components.
It has now been surprisingly found that pigment dispersants including certain carboxylic acid-functional first polymers and certain poly(ethylene oxide/propylene oxide) block second polymers in a selected ratio provide improved pigment dispersion properties which are also of benefit in inks formed therefrom.
In a first aspect of the present invention there is provided a pigment dispersant including 80-97% by weight, based on the weight of the pigment dispersant, of a first polymer including as copolymerized units 15-75% by weight, based on the first polymer weight, ethylenically unsaturated carboxylic acid-functional monomer, the polymer having a Mw from 4,000 to 16,000, and the polymer having a Tg greater than 70xc2x0 C.; and 3-20% by weight, based on the weight of the pigment dispersant, of a poly(ethylene oxide/propylene oxide) block second polymer having an Mw from 3,000 to 20,000 and an HLB value of 16 to 2.
In a second aspect of the present invention there is provided a method for dispersing a pigment in an aqueous medium including admixing an aqueous medium; a pigment; and 1-50% by weight, based on dry pigment weight, of the pigment dispersant of the present invention and subjecting the admixture to shear for a time sufficient to disperse the pigment.
In a third aspect of the present invention there is provided a pigment dispersion including a pigment dispersed in an aqueous medium and 1-50% by weight, based on dry pigment weight, of the pigment dispersant of the present invention.
In a fourth aspect of the present invention there is provided an aqueous ink including the pigment dispersion of the present invention.
The pigment dispersant of the present invention includes 80-97% by weight, based on the weight of the pigment dispersant, of a first polymer including as copolymerized units 15-75% by weight, based on the first polymer weight, ethylenically unsaturated carboxylic acid functional monomer, the polymer having a Mw from 4,000 to 16,000, and the polymer having a Tg greater than 70xc2x0 C. The first polymer includes as copolymerized units 15-75%, preferably 20-55%, by weight, based on the first polymer weight, ethylenically unsaturated carboxylic acid-functional monomer such as, for example, (meth)acrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, and maleic anhydride. A preferred first polymer includes as copolymerized units 25-40%, by weight, based on the first polymer weight, acrylic acid.
The first polymer further includes, as copolymerized unit(s), at least one monoethylenically-unsaturated monomer in addition to the carboxylic acid-functional monomer such as, for example, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, aminoalkyl (meth)acrylate, N-alkyl aminoalkyl (methacrylate), N,N-dialkyl aminoalkyl (meth)acrylate; urieido (meth)acrylate; (meth)acrylonitrile and (meth)acrylamide; styrene, xcex1-methylstyrene, or other alkyl-substituted styrenes; butadiene; vinyl acetate, vinyl propionate, or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, and N-vinyl pyrrolidone. The use of the term xe2x80x9c(meth)xe2x80x9d followed by another term such as acrylate, acrylonitrile, or acrylamide, as used throughout the disclosure, refers to both acrylate, acrylonitrile, or acrylamide. Preferred as monoethylenically-unsaturated monomer in addition to the carboxylic acid-functional monomer is a combination of styrene and xcex1-methylstyrene.
The first polymer has a Mw from 4,000 to 16,000, and the polymer has a Tg greater than 70xc2x0 C. Mw as reported herein is weight average molecular weight as determined by Gel Permeation chromatography measured vs. polystyrene standards.
The glass transition temperature (xe2x80x9cTgxe2x80x9d) of the first polymer is greater than 70xc2x0 C., the monomers and amounts of the monomers selected to achieve the desired polymer Tg range are well known in the art. Tgs used herein are those calculated by using the Fox equation (T. G. Fox, Bull. Am. Physics Soc., Volume 1, Issue No. 3, page 123(1956)). that is, for calculating the Tg of a copolymer of monomers M1 and M2,
1/Tg(calc.)=w(M1)/Tg(M1)+w(M2)/Tg(M2), wherein
Tg(calc.) is the glass transition temperature calculated for the copolymer
w(M1) is the weight fraction of monomer M1 in the copolymer
w(M2) is the weight fraction of monomer M2 in the copolymer
Tg(M1) is the glass transition temperature of the homopolymer of M1
Tg(M2) is the glass transition temperature of the homopolymer of M2, all temperatures being in xc2x0 K.
The glass transition temperatures of homopolymers may be found, for example, in xe2x80x9cPolymer Handbookxe2x80x9d, edited by J. Brandrup and E. H. Immergut, Interscience Publishers.
The free radical addition polymerization techniques used to prepare the first polymer of the pigment dispersant of this invention are well known in the art. Preferred are polymerization in concentrated solution or in bulk, preferably by a semi-continuous or a continuous process using, for example, a plug flow reactor, a CFSTR, or a hot tube reactor. Free-radical generating initiator compounds such as peroxides and hydroperoxides may be used at a level of 0-5% by weight based on total monomer weight. The monomer mixture may be added uniformly as to rate or composition. The reaction temperature is maintained at a temperature between 100xc2x0 C. and 500xc2x0 C. for a residence time of 1-30 minutes. Preferred is a reaction temperature between 150xc2x0 C. and 240xc2x0 C. for a residence time of 2.5-10 minutes.
The pigment dispersant of the present invention also includes 3-20% by weight, based on the weight of the pigment dispersant, of a poly(ethylene oxide/propylene oxide) block second polymer having an Mw from 3,000 to 20,000 and an HLB value of 16 to 32. Typically the block copolymers are solids at 25xc2x0 C. The poly(ethylene oxide/propylene oxide) block copolymer includes two or more blocks, each block being composed of polyethylene oxide or polypropylene oxide. The poly(ethylene oxide/propylene oxide) block polymers may be prepared by methods known in the art and may be mono- or di-hydroxy terminated. For example, a polyethylene oxide polymer may be reacted with propylene oxide to form a poly(propylene oxide/ethylene oxide/propylene oxide) triblock polymer. Alternatively, a polypropylene oxide polymer may be reacted with ethylene oxide to form a poly(ethylene oxide/propylene oxide/ethylene oxide) triblock polymer. Many such compositions are commercially available from BASF Corporation under the PLURONIC(trademark) trademark.
The pigment dispersant of the present invention includes 80-97% by weight, based on the weight of the pigment dispersant, of a first polymer as described hereinabove and 3-20% by weight, based on the weight of the pigment dispersant, poly(ethylene oxide/propylene oxide) block second polymer as described hereinabove. The first and second polymer are typically formed separately and admixed to provide the pigment dispersant. One or both of the first and second polymer may be in the form of a solution, preferably a solution in an aqueous medium, at the time of admixing, whether formed directly in that solution or subsequently dissolved. Alternatively, the first and second polymer may both be solids and be ground together or mixed together to form the pigment dispersant.
In the second aspect of the present invention there is provided a method for dispersing a pigment in an aqueous medium including admixing an aqueous medium; a pigment; and 1-50% by weight, based on dry pigment weight, of the pigment dispersant of the present invention and subjecting the admixture to shear for a time sufficient to disperse the pigment. By an xe2x80x9caqueous mediumxe2x80x9d herein is meant water or an homogeneous mixture which is predominantly water but which also contains amounts of water miscible solvent(s) such as, for example, alcohols, glycols, glycol ethers, glycol esters, and the like. The pigment may be at least one organic or inorganic pigment, a pigment composite, or mixtures thereof. The admixture includes 1-50%, preferably 8-25%, by weight, based on dry pigment weight, of the pigment dispersant of the present invention. The admixture is then subjected to shearing in a grinding or milling device as is well known in the pigment dispersion art. Such grinding or milling devices include roller mills, ball mills, bead mills, attrittor mills and include mills in which the admixture is continuously recirculated. The shearing of the admixture is continued for a time sufficient to disperse the pigment by which is meant herein that the objective of reducing the pigment to its primary particle size to maximize stability and color development is typically balanced against the cost in time and energy required to achieve the ultimate extent of dispersion. The time sufficient to disperse the pigment is typically dependent on the nature of the pigment and pigment dispersant and the grinding or milling device which is used and will be determined by the skilled practitioner.
The pigment dispersion which results from the method for dispersing a pigment is typically provided at 35-50% pigment, by weight, based on the weight of the pigment dispersion. The pigment dispersion is desirably stable to settling and viscosity change; long-term stability is simulated by heat-aging the dispersion at 49xc2x0 C. (120xc2x0 F.) for periods up to one month. The pigment dispersion may also include additional ingredients such as waxes, defoamers, biocides, and the like.
The pigment dispersion of this invention may be used in inks, paints, paper coatings, leather coatings, adhesives, and the like. In preferred aspect of the present invention an ink, such as, for example, an aqueous flexographic ink, may be prepared from the pigment dispersion by diluting the pigment dispersion and optionally admixing conventional ink ingredients such as waxes, defoamers, binder resins, thickeners, humectants, biocides, and the like.